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Rupture Disks And Transients


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#1

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Posted 17 July 2006 - 04:50 PM

Hello

I am trying to wrap my mind around the following scenario.

A slurry, SG=1.05 @ 160F and 3000USgpm, runs through an 18" header. From there it ties into a "distribution box" where the slurry is spread to a cluster of cyclones with a springloaded valve between each cyclone and box. The overflow of each cyclone goes into tank1, the underflow into tank2. Some distance upstream of the distribution box a 10" bypass lines tees of the 18" header and connects to tank2 (from there the underflow slurry is redirected to a different process). The process is continous.

The distribution box has a MAWP of around 170PSIg, the operating pressure is around 70PSIg. I want to put a rupture disk in the bypass line to protect the distribution box. Let's assume for now it is set to burst at 150PSIg (taking into account backpressure, manufacturing tolerance, burst tolerance etc).

The main failure scenario is a power failure which shuts the valves close. Since the valves shut immediately, I need to check whether the pressure rise in that case is high enough to burst the rupture disk.

1) Can I assume that there is a valve just downstream of the box that closes shut immediately, instead of having to deal with the geometry of the box (spherical) and having to ponder whether some pressure waves do get reflected and interfer with each other etcetc?
2) As the pressure wave travels downstream the header, and reaches the T, two things should (will?) happen. The pressure wave continues going down the main header (until it hits a pump a few hundred meters downstream), and the rupture disks bursts. When the rupture disks bursts the slurry should leave through the bypass line (given the line size is correct to relief the required flow).

So, essentially, a pressure wave goes down the main header, "telling" the slurry about a zero-velocity condition close to the distribution box. However, as that wave is travelling, the rupture disk in the bypass line bursts and the fluid upstream of the pressure wave starts flowing again? Is there some other pressure wave travelling downstream the main header when the rupture disk bursts?

I think, although the process setup looks realatively simple, the transient scenario is quite intricate. I am curious to hear what thoughts others might have about this scenario and whether I am simplifying things too much.

The attached drawing is simplified a lot (no valves shown between box and cyclones etc).

Attached Files



#2

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Posted 01 August 2006 - 10:39 AM

I ended up doing the following:

I used the resistance to flow method to establish the flow rate through the bypass line, assuming 0 velocity at its entrance and the vessel's MAWP as entrance pressure. I checked whether the bypass line could flow the required flow rate and found out it could flow the required rate.

Then, using Phil's excellent rupture disc guide, establishing the set pressure for the rupture disc was an easy one. I actually ended up with several "solutions" for the set pressure, which I will discuss with the client. The different solutions I got are based on
1) 10% manufacturing tolerance
2) 0% manufacturing tolerance
3) Different backpressure

Now the code allows that the maximum burst pressure can exceed the MAWP of the protected vessel, if that is caused by the burst tolerance. Is that could practice however to do so?

What I also do not know is, when the manufacturer stamps their disc, are they stamping in with or without taking into account the burst tolerance? My understanding was they stamp without burst tolerance, so the engineer has to be aware of that before their final go ahead. Okay, just reread the code, it seems to me that the rupture disc must be marked at a pressure within the manufacturing range. The true burst range however is obtained once the burst tolerance is accounted for.

And finally, should the sizing be done for MAWP or relief pressure? The code states that if the rupture disc is the primary relief device, it must be sized to prevent the pressure in the vessel from rising more than 10% or 3 psi, whichever is greater. In addition, its marked burst pressure shall not exceed the MAWP.

For simplicity's sake, lets look at a case with a vessel with 500psi MAWP, which shall be protected by a rupture disc with 0% manufacturing tolerance. There is also no back pressure, built up back pressure or superimposed pressure in the relief line.

According to the code, the relief pressure would need be less or equal to 550psig. However, if I would use a set pressure of 550psig for the rupture disc, its stamped burst pressure would exceed MAWP, thus violating the code. So what is that 10% or 3psi rule actually for? It seems to contradict with "the marked burst pressure shall not exceed MAWP".

So in this scenario, set pressure would be 500psi, or if I want to be supercautious 476 (taking into account BT)?

#3 pleckner

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Posted 01 August 2006 - 11:55 AM

Re-read Part 3 of my series on Rupture Disks. The stamped burst pressure cannot exceed the design pressure (or MAWP), period.

The concept of overpressure (10% or 3 psi) applies more so to pressrue relief valves than it does to ruptiure disks. For the most part, PSVs do not flow their capacity until the protected vessel pressure reaches its allowed overpressure. Code allows the vessel to build to its design pressure (or MAWP) plus an allowed overpressure.

In rupture disk sizing, we try to maintain as low a cost as possible. One way of doing this is to design for the smallest size rupture disk as possible. One way of doing this is to make the relief piping system as small as possible. This will increase the pressure build up in the vessel as the system is relieving. With a rupture disk relief system, we are still allowed to let the vessel pressure build to its design (or MAWP) plus the allowable overpressure.

It is understood by Code that tolenances have an affect on the ultimate pressure the vessel will see during relief. That is why Code specifies the allowabel tolerances. And also remember that vessels are tested at 1.3 (as a minimum) times their design pressures (or MAWP), so they have plenty of fat.

Does this make sense?

#4

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Posted 01 August 2006 - 02:30 PM

QUOTE (pleckner @ Aug 1 2006, 10:55 AM) <{POST_SNAPBACK}>
In rupture disk sizing, we try to maintain as low a cost as possible. One way of doing this is to design for the smallest size rupture disk as possible. One way of doing this is to make the relief piping system as small as possible. This will increase the pressure build up in the vessel as the system is relieving. With a rupture disk relief system, we are still allowed to let the vessel pressure build to its design (or MAWP) plus the allowable overpressure.

It is understood by Code that tolenances have an affect on the ultimate pressure the vessel will see during relief. That is why Code specifies the allowabel tolerances. And also remember that vessels are tested at 1.3 (as a minimum) times their design pressures (or MAWP), so they have plenty of fat.

Does this make sense?


It makes sense, just let me rephrase the above in my own words to see whether I was able to capture the essence of your statements.

The 10% or 3psi rule is there to tell the engineer what pressure is allowed in the protected vessel "after" the rupture disk or PSV actually started to relief. This is because there might be some over pressure building up while the vessel is reliefing, and it is at that pressure that the PSV/disc must be able to relieve the system. Makes sense now.

So, back to the 500 psi MAWP vessel (primary relief device, 0% manufacturing tolerance)
1) Relief rate is being determined based on relief pressure = MAWP+10%.
2) Rupture disk pressure is determined by stamped burst pressure < MAWP, taking into account back pressure etc.

#5 pleckner

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Posted 02 August 2006 - 06:12 AM

Not to get anal about it (well, yes), first, a correction is needed in the terminology. When you say "Manufacturing Tolerance" I assume you really mean "Manufacturing Range". The common terminology for tolerance is "Burst Tolerance".

Your understanding of the overpressure is essentially correct. The vessel will be at this overpressure at the time a PSV opens. It may or may not be at the time when a rupture disk bursts.

Yes, we calculate the required relieving rate based on relieving pressure, which is design pressure (or MAWP) + allowable overpressure.

I'm not sure what you mean by "Rupture disk pressure" unless you mean the "actaul" burst perssure. But in your case, the actual burst pressure will be essentially the stamped burst pressure. If the stamp = MAWP, then it will burst at MAWP. If the stamped < MAWP, then it will burst at less than MAWP. And yes, for this discussion I'm assuming the burst tolerance will be zero as well.

Anything else or if you need further clarification, don't hesitate to post your questions.

#6

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Posted 02 August 2006 - 09:05 AM

You are absolutely right. From re-reading my own posts I can tell that I am just beginning to understand, hence the "confusion" with the terminology.

With that said, "manufacturing tolerance" should have been "manufacturing range" and with "rupture disk pressure" I meant the "actual burst pressure" of the rupture disk.

Thanks for all the help, Phil.




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